Secondary batteries such as nickel-hydrogen cells and nickel-cadmium (Ni—Cd) cells are recently expected to be used as power sources for electric cars as well as small batteries for electrical/electronic appliances. Such a secondary battery is generally constituted of a positive electrode, a negative electrode, and a separator. Among these, the separator serves to prevent the cell from short-circuiting between the electrodes and enable ions to pass therethrough, and is required to have hydrophilicity, chemical resistance, and mechanical strength. A hydrophilic nonwoven fabric formed from a polyamide resin or the like has conventionally been known as a separator. However, this separator has insufficient resistance to chemicals (alkalis and acids). Because of this, separators comprising polyolefin nonwoven fabrics which have undergone various treatments are also known. Specifically, separators obtained by subjecting a polyolefin-based nonwoven fabric to a hydrophilizing treatment, e.g., impregnation with a surfactant, plasma treatment, grafting treatment, sulfonation treatment, or the like, have been proposed (Unexamined Published Japanese Patent Applications Nos. 4-167355 and 11-238496, etc.).
Furthermore, since the polyamide-based nonwoven fabric has amide bonds, batteries employing this as a separator show a higher degree of self-discharge than batteries employing an electrochemically inert polyolefin nonwoven fabric and have poor battery properties. In contrast, batteries employing a separator obtained by subjecting a polyolefin-based nonwoven fabric to a specific treatment cannot be regarded as fully satisfactory in self-discharge characteristics, although superior in overall battery properties to batteries employing the polyamide-based separator.
Specifically, the separator obtained by treating a polyolefin-based nonwoven fabric with a surfactant shows effective hydrophilicity in the initial state of use. However, when this separator is once immersed in water, taken out therefrom, dried, and reimmersed in water, the hydrophilicity decreases considerably. In addition, this separator is unsatisfactory in self-discharge characteristics.
Furthermore, the polyolefin-based nonwoven fabric which has undergone a plasma treatment has hydrophilic groups bonded to the substrate surface by covalent bonding and hence retains sufficient wettability even when it is immersed in water, dried once, and reimmersed in water. Namely, it is wet-dry reversible. However, in the case where this nonwoven fabric is immersed in an aqueous alkali solution having a high concentration, it is not wetted by water when it is washed with water, dried, and reimmersed in water. It is presumed that the hydrophilic but weakly adherent, interfacial layer formed on the substrate surface by the plasma treatment was peeled off upon contact with the high concentration aqueous alkali solution. This separator also is ineffective in greatly improving the inhibition of self-discharge.
In the case of the polyolefin-based nonwoven fabric which has undergone a grafting treatment, a water-soluble monomer is tenaciously bonded to a substrate by covalent bonding. However, the polyolefin treated by grafting with acrylic acid or methacrylic acid has the possibility of undergoing oxidative decomposition in a strongly oxidizing atmosphere because this polyolefin is of the carboxylic acid type. Consequently, this nonwoven fabric is used as a battery separator in limited applications.
Furthermore, the polyolefin-based nonwoven fabric which has undergone a sulfonation treatment has sulfonic groups tenaciously bonded to the substrate by covalent bonding. Consequently, this nonwoven fabric retains long-lasting hydrophilicity and functions to inhibit a battery from suffering self-discharge. However, the treatment necessitates a post-washing step.
An object of the invention is to provide a separator which eliminates the drawbacks of the conventional battery separators described above and is capable of inhibiting a battery from suffering self-discharge. The present inventors made extensive investigations on the self-discharge of batteries. As a result, it has unexpectedly been found that the self-discharge of a battery is considerably inhibited by using a separator having polycarbodiimide present thereon. The invention has thus been completed.